A. Field of the Invention
This invention relates to fuel cells.
B. Description of the Related Art
Fuel cells are energy conversion devices that use hydrogen as a fuel and oxygen as an oxidant to generate electricity without combustion and without harmful emissions. The voltage and current output of a fuel cell system depends on the number of cells in the stack, total active surface area, and efficiency. The basic process, for a single cell, is shown in FIG. 1.
Traditional fuel cell stacks are made of many individual cells 10 which are stacked together. See FIGS. 2, 3 & 4. Such fuel cells typically have a bipolar separator plate (BSP) 12 typically made of machined graphite, a membrane electrode assembly (MEA) 14, gaskets 16, 18, a fuel manifold 24, and may have oxidizer and a coolant manifolds. See FIGS. 3 and 4.
For the proper operation of fuel cells, the hydrogen gas must be sealed inside the cell and separated from the gaseous oxidant (air or oxygen). In some fuel cells, cooling is required because of the heat generated during normal operation. This heat is commonly removed from the fuel cell stacks by liquid cooling, commonly using water as a coolant.
Additionally, it is critical that the BSP 12 be in intimate, continuous electrical contact with the MEA 14.
As shown in FIGS. 2 & 4, fuel cell stacks have typically used a “filter-press” structure, where thick and heavy “end plates” 32, 34 are placed at the ends of each fuel cell stack 10 and are held together by heavy tie-rods, or bolts 38 and nuts 40, or other fasteners.
The “filter press” structure is an attempt to serve two purposes: (i) sealing the hydrogen, the oxidant, and the liquid coolant, if used, and (ii) maintaining intimate electrical contact between the BSPs 12 and the MEAs 14. see FIGS. 2 and 4. Disassembly and analysis of fuel cell stacks built by traditional methods reveals that the “filter press” arrangement performs neither function very well. Such analysis has revealed evidence of incomplete electrical contact between BSPs 12 and MEAs 14, resulting in poor electrical conduction and lower cell performance. The analysis has also shown evidence of gas and liquid leakage.
To address the difficulty in maintaining electrical connection, the inventors of the present patent have developed a system of independently acting electrical contacts that robustly maintain contact between the BSP and MEA. These contacts are described in U.S. application Ser. No. 10/068,154. That patent application is incorporated herein by reference as if set out in full.
As explained below, the present invention is a further improvement that allows even better electrical connection between the BSP and the MEA.